In a process to effect the cryogenic separation of air into at least a nitrogen-enriched product and an oxygen-rich product, it is necessary to essentially remove the carbon dioxide and other impurities (e.g., water and hydrocarbons) present in the feed air so as to prevent these impurities from freezing-out on the process equipment at cryogenic temperatures. When freeze-outs occur in a cryogenic process, it causes a loss of performance and, potentially, an unsafe operating mode. Generally, two methods are used for such impurity removal. These are temperature swing adsorption (TSA) and pressure swing adsorption (PSA).
In each of these techniques, a bed of adsorbent is exposed to a flow of feed air so that the adsorbent can adsorb the bulk of the carbon dioxide and water vapor present in the feed air. This exposure is continued for a fixed period of time which is sufficiently short in duration so as to prevent the break-through of carbon dioxide and water in the exiting treated feed air. Thereafter, the flow of feed air is shut off from the adsorbent bed and the adsorbent is exposed to a flow of regeneration gas which strips the adsorbed carbon dioxide and water from the adsorbent and, thus, regenerates it for further use. In a temperature swing adsorber, the carbon dioxide and water are driven off from the adsorbent by heating the adsorbent in the regeneration phase. In a pressure swing adsorber, the pressure of the regeneration gas is lower than that of the feed gas and the change in pressure is used to remove the carbon dioxide and water from the adsorbent. Although discussed with reference to carbon dioxide and water removal, other impurities can also be removed from the feed air by these processes, including hydrocarbons.
The adsorbent material for these TSA and PSA adsorption processes may be molecular sieves, alumina, silica gel, plus other mixed oxides, either alone or in combination with each other.
A suitable regeneration gas for these TSA and PSA adsorption processes must have the certain properties, essentially impurity-free (carbon dioxide, water, hydrocarbons); capable of being contaminated with the desorbed impurities, and capable of being operated safely and not requiring an unconventional or exotic metallurgy for the process equipment. The gas that best meets this criteria is a nitrogen-enriched stream removed from the air separation unit.
In general, regeneration of adsorption beds includes a depressurization step. The depressurization step reduces the overall bed pressure and allows strongly adsorbed components to desorb. The desorption of strongly adsorbed components is enhanced by conditions which lower the extent of adsorption, namely decrease in partial pressure and increase in temperature. The pressure reduction step helps reduce the partial pressure of the adsorbed component and thereby enhance desorption. It is also well known in the adsorption literature, that the effectiveness of purge gas is defined by actual volume of purge not moles of purge gas (Skarstrom, U.S. Pat. No. 2,944,627). This relation suggests that for a given number of moles of gas, regeneration at the lowest pressure (highest actual volume) is most effective.
However, it is occasionally desired to regenerate adsorption beds at an elevated pressure (i.e., in excess of three (3) bar(a)). For example, there is an existing commercial installation with temperature swing adsorption which uses the nitrogen-enriched gas from the air separation unit which has been compressed to a pressure of fifteen (15) bar(a). This is particularly desired when the regeneration gas effluent is used as a feed to a gas turbine. In these power generation applications, several industry-wide problems have been identified with the use of a low pressure (i.e., less than or equal to three (3) bar (a)) regeneration gas. These are: (a) the need to maintain a constant pressure drop for the regeneration gas across the bed which results in an expensive trade-off between high velocities and large diameter vessels; (b) the extra power needed to compress the desorbed water and carbon dioxide; (c) the less favorable impact of pressure drop (assumed constant) on power because of the higher pressure ratio (power=k ln(p2/p1)); and (d) the extra capital cost required to recover heat of compression in order to replace or augment the need for heat addition.
On the other hand, several industry-wide problems have, however, also been identified with the use of elevated pressure regeneration. These are: (a) because the total bed pressure is high, the partial pressure of the adsorbed impurity remains high during the desorption process, thus, rendering desorption less favorable and requiring the use of either higher regeneration temperatures or flowrates and (b) the generation of steam during the regeneration process is possible when water-laden adsorbents are regenerated at high pressure.
Because the regeneration pressure is elevated (with a consequent low actual volume of regeneration gas), the contact time of the regeneration gas in the bed can be quite high. The in-situ generated steam can then react with standard desiccants like alumina, silica gel and zeolites. This steam reacts with the desiccants and causes "aging" of the materials. In the case of alumina, this aging, or loss of dehydration performance, is caused by reaction of steam with the alumina oxide to form aluminum hydroxide. The resultant alumina hydroxide has a lower surface area and lower water capacity than "fresh" alumina. The same type of reactions occur with silica gel. In the case of zeolites, steam can react with the zeolite structure, resulting in the loss of framework aluminum which then causes loss of crystallinity and adsorption capacity of the zeolite.
Even in conventional low pressure regeneration temperature swing adsorbers, there is a gradual decrease in the capacity of the adsorbents. Although the rate of degradation is affected by many factors including regeneration temperature, the concentration of corrosive gases such as SO.sub.2, NO.sub.2, Cl.sub.2 or NH.sub.3 in the feed air, the air separation industry experience is that adsorbents are rarely replaced in less than five (5) years, and may perform satisfactorily in service for more than ten (10) years.
Temperature swing adsorption is an energy intensive process because of the need to supply heat to the regenerating gas. The temperatures needed for the regeneration gas are typically high, i.e., 150.degree. C. to 200.degree. C., which places demands on the system engineering which, in turn, increase costs. Most literature concerning temperature swing adsorption is aimed at reducing the heat input to the adsorber system. There are numerous references which teach methods for improving the operation of adsorption beds by such things as improved bed designs (e.g., U.S. Pat. Nos. 4,249,915 and 4,472,178), adsorbent material, combination and placement of adsorbent material, cycle times (heating, cooling, and purge) (e.g., von Gemmingen, U., "Designs of Adsorptive Dryers in Air Separation Plants", Reports on Technology, 54/1994, (Linde)), and regeneration heat-up temperature, regeneration cool-down temperature, temperature pulsing (e.g., U.S. Pat. No. 5,137,548 and U.S. Pat. No. 4,541,851).
All these references refer to regeneration with a near atmospheric gas (i.e., low pressure regeneration), either explicitly or implicitly through a description of the steps in the cycle and the order in which they occur: (a) adsorption, (b) depressurization, (c) regeneration and (d) repressurization. No information has been uncovered which teaches or suggests the use of a compressed air separation unit product as regeneration gas for the adsorbent.
Acharya and Jain (1995) in "Recent Advances in Molecular Sieve Unit Design for Air Separation Plants", Separation Science and Technology, 30(18), pp 3489-3507 describe the advantages of mixed alumina/13X beds. They teach that the use of alumina for water adsorption is preferable for many reasons, one of which is that alumina is "a more resilient material than molecular sieve".
With respect to zeolites, the prior art which suggests that reaction of zeolites, particularly low Si/Al ratio zeolites, with water causes crystal structure damage. For example, it has been shown that exposure of zeolite NaX (13X) to steam at 350.degree. C. results in loss of crystal structure and adsorption capacity (i.e., F. Wolf, H. Fuertig, G. Nemitz, Chem. Tech., Leipzig, 19:83, 1967). In contrast, zeolite Y, which has the same crystal structure as X, but a higher Si/Al ratio, retains its structure when exposed to water vapor at 410.degree.C.
Other references include U.S. Pat. No. 4,541,851 that teaches a temperature swing adsorption process in which the heat pulse is consumed in desorbing both the more strongly and weakly adsorbed components from the adsorbent. U.S. Pat. Nos. 4,249,915 and 4,472,178 teach an adsorption process in which water and carbon dioxide are removed from atmospheric air by adsorption in separate beds with the water laden bed being regenerated by pressure swing adsorption in a relatively short operating cycle while the carbon dioxide laden bed is regenerated thermally at considerably longer time intervals.